Google and Yahoo require you to enter your username first, then password. Why is this? [duplicate]

1. Why is Yahoo asking for my password again?
2. What is the reason why there is need to have a username and password in accessing the system?
3. Why do I have to keep signing into my Yahoo email account?
4. Why do I have to keep signing in to my email?

Why is Yahoo asking for my password again?

If we've noticed any unusual activity on your account or if you're using a password that's old or not strong enough, we'll ask you to change it. By creating a new password, your personal info remains secured from intruders and imposters.

What is the reason why there is need to have a username and password in accessing the system?

Passwords provide the first line of defense against unauthorized access to your computer and personal information. The stronger your password, the more protected your computer will be from hackers and malicious software. You should maintain strong passwords for all accounts on your computer.

Why do I have to keep signing into my Yahoo email account?

Even if you choose to stay signed in when you log in to Yahoo, Yahoo still might prompt you to log in each time you check your Yahoo Mail. If this is the case, then the browser isn't saving login cookies, which are bits of data that let Yahoo know you're a return visitor.

Why do I have to keep signing in to my email?

The cookie file is corrupted - Clear out the cookie file on your desktop or mobile device. You cleared cookies manually - If the settings weren't changed, once you sign in, you'll stay signed in. A security software is clearing cookies - Disable the program and change its settings to stay signed in.